Regarding iPod models that do not have activation lock (i.e. the Classic, nano and shuffle) - you can simply delete the music that was on it.
If you use iTunes to Sync your music to the iPod, part of the procedure will ask you if you want to delete the previous owner's music that is currently on it (yes, you do). I assume that the procedure on a Mac using Finder* instead of iTunes has the same option somewhere.
My iPod Classic gave up many years ago, so I'm not sure whether my memory serves me me enough but if the Classic has a PIN code on it, you need to know what that is so that you can change it or turn it off. (This is not activation code, nor is it as draconian as the lock code on later iPods. I seem to recall that there might be an option to lock the iPod via a four-digit code, but I may be wrong)
* Lastly, Finder.
From Mac OS Catalina (10:15) or later, there is no iTunes. Instead, one uses Finder. Older Macs, using OS Mojave (10:14) or earlier, still use iTunes,as does Windows.
When you buy a second-hand iPod from a shop, I would expect that shop to provide some sort of warranty. Not thew Apple warranty of course, but their own guarantee that the device will be useable by you for a minimum period of time after you purchase it. (Subject to local laws, it might be 30, or 90 days for example.) So if you got it home and you found it locked, you should be able to get your money back from that shop.
